This invention relates to the field of secondary surveillance radar (SSR) systems, which are systems designed to operate in conjunction with primary radar for precisely locating aircraft by transmitting information to them and processing their replies. More particularly, this invention relates to an SSR system in which monopulse processing is used to obtain superior angle estimation of azimuth bearings of aircraft.
By monopulse processing is meant techniques which determine aircraft azimuths on a pulse-by-pulse basis so as to permit highly accurate azimuth estimates to be made on a single reply per scan. These techniques permit a very high degree of accuracy in determining azimuth. Moreover, monopulse techniques permit secondary surveillance radar operation at a greatly reduced pulse repetition frequency compared to that required by more conventional azimuth measurement techniques.
The present invention is an improvement on systems described in U.S. Pat. No. 4,689,623, assigned to the assignee of the present invention. For a complete background on pertinent radar systems, reference may be made to a number of references cited in U.S. Pat. No. 4,689,623. While it will be appreciated that these known systems have various merits and advantages, they do not provide a completely efficient, relatively simple and cost effective SSR system.
Although the system described in the aforenoted U.S. Pat. No. 4,689,623 is considered reasonably cost effective, it has been found to have certain limitations. For example, it does not provide a sufficiently bounded identification of the maximum unambiguous off-boresight azimuth (OBA) and, at the same time, reduce sensitivity to Omni channel variations. Stated another way, excessive Omni channel signal variations will permit the Omni channel to exert an undesired RSLS gate action (possible erroneous RSLS suppression).